Exploring the reactivity of alpha-triorganylsilyl alpha-diazo esters : C-H activation, insertion reactions, and rearrangements
The metal-catalyzed reactions of Alpha-triorganylsilyl Alpha-diazo esters have been studied, with an emphasis on: · Dirhodium(II)-catalyzed insertions of Alpha-triorganylsilyl Alpha-diazo esters into activated C-H bonds. · Dirhodium(II)-catalyzed reactions of -triorganylsilyl Alpha-diazo esters with various acyclic and cyclic carbonyl precursors. · The syntheses of Alpha-silyl-substituted Alpha-keto esters, their corresponding hydroxy esters and hydroxy acids, in addition to investigating dirhodium(II)-catalyzed O-H insertion reactions. Insertion of benzyl and ethyl 2-triorganylsilyl 2-diazoacetates into activated C-H bonds adjacent to oxygen, such as in tetrahydrofuran, was possible using dirhodium(II) acetate , albeit in moderate yields (up to 50% yield). When the chiral Davies catalyst was used, benzyl 2-tetrahydro-2-furanyl-2-trimethylsilylacetate was obtained in 58% ee. Ring expanded products were isolated from the reaction of benzyl Alpha-triorganylsilyl Alpha-diazoacetate with oxetane, attributed to a Stevens rearrangement. The dirhodium(II)-catalyzed reaction of Alpha-triorganylsilyl Alpha-diazoacetates proceeded well with various acyclic and cyclic ketones, affording dioxolanones in up to 98% yield. The effect of the structure of the diazo compound on the formation of the dioxolanones was investigated, and various novel silyl-substituted diazo precursors were synthesized for this purpose. A silylated enol ether competes with the formation of a dioxolanone, depending on the electronic and steric properties of the ester functionality. Two routes for the synthesis of Alpha-silyl-substituted Alpha-hydroxy acids were envisioned, comprising dirhodium(II)-catalyzed oxygen transfer and dirhodium(II)-catalyzed O-H insertions. Reactions of Alpha-triorganylsilyl Alpha-diazo esters with propylene oxide afforded 2-silyl-2-oxoacetates in up to 95% yield. These were converted into their corresponding enantiomerically-pure hydroxy acids via reduction using NaBH4, enantiomer separation by preparative HPLC, and palladium-catalyzed hydrogenolytic debenzylation. Alternatively, asymmetric reduction using (R)-Alpine Borane was effective in achieving 2-silyl-2-oxyacetates in up to 91% ee. Various alcohols were effectively inserted into Alpha-triorganylsilyl Alpha-diazo esters, generating 2-silyl-2-oxyacetates in up to 97% yield. Insertion of chiral (R)-and (S)-phenylethanols, followed by separation of the diastereomers, and palladium-catalyzed reduction using molecular hydrogen, made it possible to isolate enantiomerically-pure Alpha-silyl-substituted Alpha-hydroxy acids, circumventing the need for preparative HPLC separation. The best diastereomer ratio (dr) of 91:9 obtained to date was achieved with (1R,2S,5R)-(-)-8-phenylmenthyl (triethylsilyl)diazoacetate and the novel (S)-tetrakis{[2.2]PCp-4-carboxy}dirhodium(II) catalyst in O-H insertions. Finally, enantomerically-pure Alpha-silyl-substituted Alpha-hydroxy acids were tested as ligands in asymmetric catalysis.
info:eu-repo/classification/ddc/540, Ester, Diazoverbindungen, Silylgruppe, Einschiebungsreaktion, Rhodiumkomplexe, Katalysator, Chemie, rhodium, carbenoids, diazo ester, C-H insertion, carbonyl ylides, O-H insertion
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